When austenitic stainless steel pipe sections are joined at their edge surfaces by welding or similar fusing technique, the pipe wall areas immediately adjacent to the weld are caused to heat up. This heating of some types of austenitic stainless steel during welding is known to result in a metallurgical change involving precipitation of carbides at the grain boundaries. In addition, welding causes residual tensile stresses to develop in the immediate region of the weld. The combination of these elements in the presence of certain corrosive environments can promote cracking in the weld region of the pipe wall.
Recent installations generally use low carbon stainless steels, as well as stress redistribution techniques, to minimize the susceptibility of such welded pipes to stress corrosion cracking. However, the problem persists due, in part, to the types of stainless steel originally used and the welding methods employed. The occurrence of stress corrosion cracking in such installations has presented serious problems. Failure to correct the condition can lead to pipe leaks and the attendant downtime for pipe repair.
Repair procedures have been used in the past that require the removal and replacement of an entire pipe section in such an installation. A repair of that magnitude often entails long down time periods and is consequently expensive to carry out. Further, the installation of the new pipe section requires two separate butt welds, i.e. one more than the defective weld that is replaced. Aside from the additional labor required, each such additional weld lengthens the down time of the installation due to the time required for welding as well as the time for additional weld inspection required both when the repair is made and in future in-service inspections. Thus, if it is possible to do so within the constraints imposed by safety, inspection and other operational requirements of the installation, it is clearly preferable to replace the defective weld with a single new weld rather than to install a pipe section.
In certain installations, piping must be operated at high pressures. Such piping may also carry dangerous substances. In such applications, any pipe failure may represent an unacceptably high risk to workers, to the public, or to the continued integrity of the installation of which the piping is a part. Thus, when a pipe joint is repaired, the repaired joint must be of a quality and integrity which is in conformance with pertinent industry standards and which is no less than the quality and integrity of the remainder of the pipe.
Various solutions have been proposed to the problem of repairing austenitic stainless steel pipe weld joints in which stress corrosion cracking has occurred. One repair procedure known in the art calls for build-up welding on top of the defective weld zone to be repaired. Typically this procedure is carried out while maintaining a liquid coolant in contact with the inside surface of the pipe in the vicinity of the aforesaid weld zone. Cooling at the pipe interior during exterior welding results in the redistribution of stress in the weld region, thereby placing the interior surface of the pipe in compression. Thus, the pipe becomes more resistant to stress corrosion cracking. However, the beneficial effects of the cooling procedure may be limited where the pipe wall is so thick as to prevent the required temperature gradient from being developed or existing cracks in the pipe wall extend beyond the region placed in compression. Further, the procedure fails to correct such stress corrosion cracking as has already occurred and merely welds fusible material on top of the defective weld joint.
Another known weld repair procedure is disclosed in U.S. Pat. No. 4,234,119. In accordance with one embodiment of the procedure outlined in that patent, all but the root layer of the original weld is removed. This is followed by rewelding while liquid coolant is maintained in contact with the inside pipe surface at the weld zone. It is stated in the patent that while the effect of cooling in this procedure is similar to that described above, the technique is further effective in eliminating existing stress corrosion cracks. Experience has, however, shown this may not be borne out in practice. By confining the repair to the original weld without replacing the susceptible weld heat-affected zones on opposite sides of it and completely fusing the root layer, such a technique leaves the integrity of the repaired joint in doubt and further sensitizes the original weld heat-affected zones to stress corrosion.
Other techniques are known in the art which redistribute stress in the weld region. However, most of these techniques only serve to arrest existing stress corrosion cracking in the pipe, without repairing or replacing the weld where such cracking has already occurred.
To deal effectively with the problems outlined above, it is necessary to remove not only the weld, but also the adjacent defective heat-affected zones and to replace them with a new joint having a stress distribution that furthers resistance to stress corrosion cracking. However, the removed portions leave a relatively large gap in the pipe, which is difficult to bridge with an ordinary weld. Though it is known in the art to use spacer strips, backing strips, or fusible metal inserts during welding in order to facilitate the joining of two pieces of metal across a gap, the problem of the large gap in a pipe where the inside surface is not accessible after welding is not addressed. As a consequence, there currently exists no effective, simple and inexpensive method for repairing or replacing a defective circumferential butt weld joint in an austenitic stainless steel pipe, which is capable of bridging a relatively large gap and which provides a new weld joint that resists corrosion cracking and which is of an integrity at least equal to that of the remainder of the pipe so as to conform to applicable industry safety standards.